The virus, which is spread by mosquito bites and likely by sex, commonly causes symptoms such as fever, rash, joint pain or red eyes. There is no vaccine or treatment, and while infections are usually mild, it has been linked to birth defects in pregnant women.

One of the most common of those defects is microcephaly, a defect that causes babies to be born with an abnormally small head. These babies often have a brain that is underdeveloped.

The link between Zika and microcephaly is still being investigated, but authorities are warning pregnant women to avoid travelling to countries most affected by the virus, and women living in some of these countries to try to delay getting pregnant until 2018.

Microcephaly symptoms and causes

In babies born with this condition, it can cause developmental delays, seizures, or problems with speaking, sitting, walking, balance, feeding, hearing, and vision, according to the CDC. The symptoms, which are often lifelong, range from mild to severe. In some cases, the condition can be life-threatening.

Microcephaly can be diagnosed by ultrasound late in the second trimester or early in the third trimester of pregnancy, or after the baby is born by measuring its head and comparing it to growth charts.

We don’t know exactly what causes babies to develop this defect, but it can be related to genetics, infections, severe malnutrition, or exposure to alcohol, drugs, or other toxic chemicals.

Why could Zika be linked with microcephaly?

Ordinarily, microcephaly is pretty rare. About 2 to 12 babies per 10,000 are born with the condition in the US. In Brazil, where the Zika outbreak has been especially severe, microcephaly cases before the outbreak were about 0.5 cases for every 10,000 births. But in the second half of 2015, that number jumped to about 20 cases per 10,000 live births, according to a CDC report.

In the past few months, researchers have found genetic material from the Zika virus in the brains of infants born with microcephaly in Brazil, which suggests the virus can cross over the placenta and infect the baby’s nervous system, STAT reports.

However, a pregnant woman who gets the virus will not necessarily have a child with microcephaly, and experts don’t know the exact risk.

STAT reports that most of the Zika-infected women giving birth to babies with microcephaly contracted the virus during their first trimester of pregnancy, although they may still be at risk into their second trimester. Most congenital infections happen during the second and fourth months of pregnancy, when the most brain cells are being formed.

According to the CDC, Zika usually only remains in the blood of an infected person for a few days to a week, so it cannot infect a baby that is born after it has been cleared. While experts currently don’t think there’s a risk that Zika will cause birth defects in future pregnancies, women who have had the virus should try to consult their doctors before becoming pregnant.